Metal and metalloid pollutants such as arsenic (As), cadmium (Cd), chromium (Cr), lead (Pb), mercury (Hg), and zinc (Zn), can adversely affect the health of millions of people worldwide. Arsenic, for example, is toxic and carcinogenic. The metal and metalloid contaminated soil, sediment, and water supplies are major sources of contamination in the food chain. Metal and metalloid poisoning can occur via ingestion of contaminated drinking water and food. Industrial pollution and agricultural practices including the use of metal and metalloid-containing pesticides, herbicides, fertilizers, and wood preservatives, as well as irrigation with contaminated groundwater, and mining have significantly increased metal and metalloid contamination in agricultural soil. There is global concern regarding arsenic contamination in drinking water and soil, particularly on the Indian subcontinent where more than 450 million people are at risk for arsenic poisoning.
There are many different ways metal and metalloid pollutants can enter the food chain. Plants grown in contaminated soil can accumulate high levels of metal and metalloid pollutants in roots, shoots, and grain. Metal and metalloid pollutant uptake by plants may play an important role in the introduction of these pollutants into the food chain, for example, by the direct ingestion of contaminated grain. In addition, contaminated straw that is used as cattle feed may have adverse health effects on cattle and may result in increased metal and metalloid exposure in humans via a plant-animal-human pathway. There is, therefore, concern regarding the accumulation of metal and metalloid pollutants in meat and dairy products as well as in agricultural crops and vegetables.
In addition, metal and metalloid pollutants are phytotoxic and cause significant loss in crop yields. For example, arsenate is a phosphate analog and competes with phosphate for uptake in plants causing the inhibition of phosphate and other nutrients. Thus, arsenic contamination is an agricultural concern. A plant that is resistant to metal and metalloid pollutants and can accumulate a large biomass despite the presence of metal and metalloid pollutants will be advantageous as a biofuel plant. Such a plant could be grown on contaminated, but otherwise arable, land.
Metals and metalloids are often present in the environment in different ionic forms. With respect to arsenic, the arsenate oxyanions, HAsO42− and H2AsO4−, are the most prevalent forms of arsenic in surface soil, water, and within cells, and these oxyanions contain arsenic in the pentavalent state [As(V)]. Arsenite, which at neutral pH contains arsenic in the trivalent oxidation state [As(III)] and likely as the acid HAs32−, is highly reactive and readily forms As(III)-thiol complexes. Plants use arsenate reductases to detoxify arsenic by reducing As(V) to As(III), which is subsequently detoxified via forming complexes with thiol-reactive peptides such as γ-glutamylcysteine (γ-EC), glutathione (GSH) and phytochelatins (PCs). It is suggested that these As(III)-thiol complexes are then sequestered into vacuoles by glutathione-conjugating pumps. It is further believed that plants trap arsenite in below ground tissues in order to prevent access to aboveground reproductive tissues to prevent possible mutagenic consequences.
Because the binding of As(III) by the thiol-reactive peptides is stoichiometric, As(III) in excess of the binding capacity of the thiol-reactive peptides may not be effectively complexed and detoxified. Accordingly, there is a need to identify novel As(III) binding polypeptides. Similarly, there is a need to identify metal and metalloid binding polypeptides in order to develop improved crops that are resistant to these pollutants.